Electrolysis cell for restoring the concentration of metal ions in electroplating processes

ABSTRACT

It is described an electrolysis cell wherein the anodic dissolution of metals is carried out, in particular of metals characterised by a relatively high oxidation potential, such as copper, or metals with high hydrogen overpotential, for example tin, aimed at restoring both the concentration of said metals, and the pH in galvanic baths used in electroplating processes with insoluble anodes. The cell of the invention comprises an anodic compartment, wherein the metal to be dissolved acts as a consumable anode, and a cathodic compartment, containing a cathode for hydrogen evolution, separated by a cation-exchange membrane. The coupling of the cell of the invention with the electroplating cell allows a strong simplification of the overall process and a sensible reduction in the relevant costs.

This application is a 371 of PCT/EP02/07182 filed Jun. 28, 2002.

DESCRIPTION OF THE INVENTION

The processes of galvanic electroplating with insoluble anodes areincreasingly more widespread for the considerable simplicity of theirmanagement with respect to the traditional processes with consumableanodes, also due to the recent improvements obtained in the formulationof dimensionally stable anodes for oxygen evolution both in acidic andin alkaline environments. In the traditional processes of galvanicplating, the conductive surface to be coated is employed as the cathodein an electrolytic process carried out in an undivided cell wherein theconcentration of the metal ions to be deposited is kept constant bymeans of the dissolution of a soluble anode under different forms(plates, shavings, spheroids, and so on).

The positively polarised anode is thus progressively consumed, releasingcations which migrate under the action of the electric field and depositon the negatively polarised cathodic surface. Although this process isalmost always advantageous in terms of energetic consumption, beingcharacterised by a reversible potential difference close to zero, somedefinitely negative characteristics make it inconvenient especially whencontinuous deposited layers having very uniform thickness are desired;the most evident of such characteristics is the progressive variation inthe interelectrodic gap due to the anode consumption, usuallycompensated by means of sophisticated mechanisms. Furthermore, theanodic surface consumption invariably presents a non fully homogeneousprofile, affecting the distribution of the lines of current andtherefore the quality of the deposit at the cathode.

In most of the cases, the anode must be replaced once a consumption of70-80% is reached; then, a new drawback arises, due to the fact that itis nearly always necessary to shut-down the process to allow for thereplacement, especially in the case, very frequent indeed, that theanode be hardly accessible. All of this implies higher maintenance costsand loss of productivity, particularly for the continuous cyclemanufacturing systems (such as coating of wires, tapes, rods, bars andso on).

For the above reasons, in most of the cases it would be desirable toresort to an electroplating cell wherein the metal to be deposited isentirely supplied in ionic form into the electrolyte, and wherein theanode is of the insoluble type, with a geometry which can be optimised,so as to fix the preferred interelectrodic gap to guarantee a qualityand homogeneity of the deposit appropriate for the most criticalapplications, suitable for continuous operations

For this purpose, as the vast majority of the galvanic applications iscarried out in an aqueous solution, the use of an electrode suitable towithstand, as the anodic half-reaction, the evolution of oxygen, isconvenient. The most commonly employed anodes are constituted of valvemetals coated with an electrocatalytic layer (for instance noble metaloxide coated titanium), as is the case of the DSA® anodes commercialisedby De Nora Elettrodi S.p.A, Italy.

To maintain a constant concentration of the ion to be deposited in theelectrolytic bath, it is necessary however to continuously supply asolution of the same to the electroplating cell, accurately monitoringits concentration. Obtaining the metal in a solution may be a problem insome cases, in particular, for the majority of the galvanicapplications, the added value of the production is too low to allow theuse of oxides or carbonates of adequate purity, and cost considerationsdemand to directly dissolve the metal to be deposited in an acidicsolution.

The direct chemical dissolution of a metal is not always a feasible oreasy operation: in some cases of industrial relevance, for instance inthe case of copper, simple thermodynamic considerations indicate that adirect dissolution in acid with evolution of hydrogen is not possible,as the reversible potential of the couple Cu(0)/Cu(II) is more noble(+0.153 V) than the one of the couple H₂/H⁺; for this reason, the bathsfor copper plating are often prepared by dissolution of copper oxide,that nevertheless has a cost which is prohibitive for the majority ofthe applications of industrial relevance.

In other cases it is instead a kinetic type obstacle which makes thedirect chemical dissolution problematic; in the case of zinc, forexample, even if the reversible potential of the couple Zn(0)/Zn(II)(−0.76 V) is significantly more negative than the one of the coupleH₂/H⁺, the kinetic penalty of the hydrogen evolution reaction on thesurface of the relevant metal (hydrogen overpotential) is high enough toinhibit its dissolution, or in any case to make it proceeding atunacceptable velocity for applications of industrial relevance. Asimilar consideration holds true also for tin and lead. This kind ofproblem may be avoided by acting externally on the electric potential ofthe metal to be dissolved, namely carrying out the dissolution in aseparate electrolytic cell (dissolution or enrichment cell) wherein saidmetal is anodically polarised so that it may be released in the solutionin ionic form, with concurrent evolution of hydrogen at the cathode. Thecompartment of such cell must be evidently divided by a suitableseparator, to avoid that the cations released by the metal migratetowards the cathode depositing again on its surface under the effect ofthe electric field. The prior art discloses two different embodimentsbased on said concept; the first one is described in the European Patent0 508 212, relating to a process of copper plating of a steel wire inalkaline environment with insoluble anode, wherein the electrolyte,based on potassium pyrophosphate forming an anionic complex with copper,is recirculated through the anodic compartment of an enrichment cell,separated from the relative cathodic compartment by means of acation-exchange membrane. Such device provides for continuouslyrestoring the concentration of copper in the electrolytic bath, but thecupric anionic complex formed in the reaction alkaline environmentinvolves some drawbacks. In particular, the copper released into thesolution in the enrichment cell is mostly but not totally engaged in thepyrophosphate complex. The fraction of copper present in cationic form,even if small, binds to the functional groups of the membrane itselfmaking its ionic conductivity decrease dramatically. A further fractiontends then to precipitate inside the membrane itself in the form ofhydrate oxide crystals, extremely dangerous for the structural integrityof the membrane itself.

Finally, in EP 0 508 212 an unwelcome process complication is madeevident, as the electroplating cell tends to be depleted of hydrogenions (consumed at the anodic compartment), which must be re-establishedthrough the addition of potassium hydroxide formed in the catholyte ofthe enrichment cell. Such re-establishment of the alkalinity requires acontinuous monitoring, implying an increase in the costs both of thesystem and its management.

In those cases where the matrix to be coated inside the electroplatingcell makes it possible, it may be convenient carrying out the process inan acidic environment rather than in an alkaline environment. In thisway, the metal involved in the process is in any case entirely presentin the cationic form but the possibilities that it may either bind tothe functional groups of the membrane in the dissolution cell orprecipitate inside the same, are drastically reduced. The use of anacidic bath, as an alternative to the alkaline bath, is foreseen in asecond embodiment of the prior art, described in the internationalpatent application WO 01/92604 whose content is incorporated herein as areference. In said embodiment, the separator used in the dissolutioncell is an anion-exchange membrane, and in principle there is nolimitation to the use of acidic or alkaline baths, as disclosed in thedescription. The process of WO 01/92604 has the advantage of beingcompletely self-regulating; however, the industrial applications carriedout so far according to the teachings of WO 01/92604 relate to the usein alkaline environment, even if in principle the process could belikewise applied to an acidic bath. In fact, although the recentdevelopments in the field of anion-exchange membranes may prospectfuture improvements in this direction, today said membrane exhibit anunsatisfactory selectivity in acidic environments as concerns anionmigration, which ideally should be nil, with respect to cationmigration. This situation constitutes quite an undesirable limitation,as the use of acidic baths is sometimes necessary; in the first place,in some cases the alkaline baths are extremely toxic both for man andthe environment (as in the case of cyanide baths, which constitute themost common types of alkaline baths for many metals), in the secondplace, the acidic baths are less subject to metal precipitation insidethe membranes and permit to operate at higher current densities withrespect to alkaline baths, wherein as already said, the metal species,being present as an anionic complex, is subject to severe limitations ofdiffusive type. Further, in many cases, it is convenient inserting thedissolution cells in existing galvanic plants, where previouslydissolution methods, obsolete or less convenient, were utilised, such asfor examples, the dissolution in the acidic bath of oxides or carbonatesof the metal. In these cases, usually it is not permitted to change thetype of bath, especially due to considerations of corrosion stability ofthe pre-existing materials; therefore, in those cases where acidic bathswere used, it may be impossible integrating a dissolution cell suitablefor operating in an alkaline environment.

It is therefore necessary to identify an enrichment cell configurationsuitable for coupling with metal electroplating cells capable ofoperating with acidic baths and of overcoming the drawbacks of the priorart. It is further necessary to detect a process for the operation of adissolution cell coupled to a metal electroplating cell capable ofoperating in acidic baths in a substantially self-regulated way.

The present invention is aimed at providing an integrated system ofgalvanic electroplating cell of the insoluble anode type hydraulicallyconnected with a dissolution or enrichment cell, overcoming thedrawbacks of the prior art, in particular exploiting the non completeselectivity for the metallic cation/hydrogen ion transport, typical ofcation-exchange membranes. In particular, the present invention isdirected to an integrated system of galvanic electroplating cell of theinsoluble anode type hydraulically connected to an enrichment cell,which may be operated with acidic electrolytes, characterised in thatthe balance of all the chemical species is self-regulating, and that noauxiliary supply of material is required except the possible addition ofwater.

The invention consists in an insoluble anode electroplating cellintegrated with a two-compartment enrichment cell fed with an acidicelectrolyte divided by at least one separator consisting of acation-exchange membrane. In a preferred embodiment, the twocompartments of the enrichment cell may act alternately as anodic orcathodic compartments. In the electroplating cell, the metal isdeposited from the corresponding cation onto a cathodically polarizedmatrix and at the same time oxygen is evolved at the anode which act asa counter-electrode, and consequently acidity is developed.

The dissolution or enrichment cell provides in a self-regulating way,for restoring the deposited metal concentration and at the same timeneutralises the acidity formed in the electroplating cell. Saidself-regulation is permitted by the fact that, under givenelectrochemical and fluid dynamic operating conditions the ratio betweenmetal ions and hydrogen ions migrating through the cation exchangemembrane in the enrichment cell is also constant. In particular, themetal whose concentration is to be restored is dissolved in the anodiccompartment of the enrichment cell and recirculated to theelectroplating cell; a fraction of the metal (typically in the range of2-15% of the total current, depending, as aforesaid, on the processconditions and nature of the cation) migrates under the electric fieldeffect through the cation-exchange membrane, without howeverprecipitating inside the same or blocking the functional groups of themembrane itself due to the acidic environment. The metal fractionmigrating through the ion-exchange membrane deposits onto the cathode ofthe enrichment cell, from where it will be recovered in the subsequentcurrent potential reversal cycle of the two compartments. The remainingcurrent fraction (85-98% of the total current) is directed to thetransport of hydrogen ions from the anodic compartment to the cathodiccompartment of the enrichment cell. The hydrogen ions discharge at thecathode, where hydrogen is evolved; accordingly, as the anolyte of theenrichment cell is electrolyte of the electroplating cell, in theenrichment cell also the consumption of the excess acidity produced inthe electroplating cell takes place. To achieve a stationaryself-regulating condition it is only necessary to apply an excesscurrent density to the enrichment cell with respect to theelectroplating current, so that the metal dissolved at the anode isequivalent to the sum of the metal deposited in the electroplating celland the metal migrating through the membrane and re-deposited at thecathode of the enrichment cell.

The invention will be more readily understood making reference to thefigure, which shows the general layout of the process for the depositionand the enrichment of a generic metal M present in the acidic bath inthe form of a cation with a charge z+.

Making reference to FIG. 1, (1) indicates the continuous electroplatingcell with insoluble anode, (2) indicates the enrichment cellhydraulically connected to the same. The described electroplatingtreatment refers to a conductive matrix (3) suitable for undergoing theplating process for the metal deposition under continuous cycle, forexample a strip or a wire; however, as it will be soon evident from thedescription, the same considerations apply to pieces subjected todiscontinuous-type operation. The matrix (3) is in electrical contactwith a cylinder (4) or equivalent electrically conductive and negativelypolarised structure. The counter-electrode is an insoluble anode (5),positively polarised. The anode (5) may be made, for example, of atitanium substrate coated by a platinum group metal oxide, or moregenerally by a conductive substrate non corrodible by the electrolyticbath under the process conditions, coated by a material electrocatalytictowards the oxygen evolution half-reaction. The enrichment cell (2),having the function of supplying the metal ions consumed in theelectroplating cell (1), is divided by a cation-exchange membrane (6)into a cathodic compartment (9) provided with a cathode (7) and ananodic compartment (10), provided with a soluble anode (8) made of themetal which has to be deposited on the matrix to be coated (3). Theanode (8) may be a planar sheet or another continuous element, or anassembly of shavings, spheroids or other small pieces, in electriccontact with a positively polarised permeable conductive confining wall,for instance a web of non corrodible material. In a preferred embodimentof the invention, the anodic and cathodic compartments may beperiodically reversed acting on the polarity of the electrodes and onthe hydraulic connections; therefore the electrodic geometry must besuch as to permit the current reversal.

The anodic compartment (10) is fed with the solution to be enrichedcoming from the electroplating cell (1) through the inlet duct (11); theenriched solution is in turn recirculated from the anodic compartment(10) of the enrichment cell (2) to the electroplating cell (1) throughthe outlet duct (12). In the case of an electroplating in acidicenvironment of metal M from the cation M^(z+), the process occursaccording to the following scheme:

-   -   conductive matrix (3) M^(z+)+z e⁻→M    -   insoluble anode (5) z/2 H₂O→z/4 O₂+z H⁺+z e⁻

The solution depleted of metal ions M^(z+)and enriched in acidity (forthe anodic production of z H⁺), as afore said, is circulated through theduct (11) in the anodic compartment (10) of the enrichment cell (2),wherein a soluble anode (8) made of positively polarised M metal, isoxidised according to:(1+t)M→(1+t)M^(z+)+(1+t)z e ⁻and the excess acidity is neutralised through the transport, shown inFIG. 1, of hydrogen ions from the anodic compartment (10) to thecathodic compartment (9), of the enrichment cell (2).

Such migration of hydrogen ions is made possible by the fact that theseparator (6) selected to divide the compartments (9) and (10) is acationic membrane; the driving force supporting the same is the electricfield, to which the contributions of osmotic pressure and diffusion addup.

The hydrogen ions migrating through the membrane (6) restore the pH ofthe bath circulating-between the anodic compartment (10) of theenrichment cell (2) and the electroplating cell (1), without howeveraffecting that of the cathodic compartment (9) of the enrichment cell(2), where they are discharged at the hydrogen evolving cathode. Not allof the electric current flowing in the enrichment cell (2) is directedto the transport of hydrogen ions; as shown in the FIGURE, a minorfraction of the same is necessarily dissipated in the transport of themetal ion M with a charge z+through the membrane (6). The ratio betweenthe portion of the effective current used for the hydrogen ion transportand the total current is defined as the hydrogen ion transport numberand it depends on the equilibrium, which is a function of theconcentrations of the two competing ions, on the nature of the metalcation, on the current density and on other electrochemical and fluiddynamic parameters, which are usually fixed. A hydrogen ion transportnumber comprised between 0.85 and 0.98 is typical of the mainelectroplating process in acidic baths, for example copper and tinelectroplating. The metal cation transported through the membrane (6) ofthe enrichment cell (2) deposits onto the cathode (7). Therefore thetransport of metal M is a parasitic process, which causes the decreaseof the overall current efficiency of the enrichment cell (2), defined bythe ratio 1/(1+t), and in principle also a loss of the metal to bedeposited. This last inconvenience however may be overcome by periodiccurrent reversals whereby the metal deposited at the cathode (7) isre-dissolved by operating the latter as an anode. It is thereforeconvenient making an accurate choice of the construction material forthe cathode (7), which must be fit for operating as an anode, even iffor short periods, without corroding. Therefore, rather than nickel andalloys thereof, which are traditional materials for cathodes inelectrolytic cells, valve metals (preferably titanium and zirconium) andstainless steel, will be adopted (for example AISI 316 and AISl 316 L),optionally coated by a suitable conductive film according to the priorart teachings.

In order to make the cathodic (9) and anodic (10) compartments of theenrichment cell (2) temporarily interchangeable, it is convenient to actalso on the hydraulic connections between the two cells (1) and (2). Inparticular, when the polarity of the enrichment cell (2) is reversed,the ducts (11) and (12) must be switched to the original cathodiccompartment (9), which upon current reversal becomes the anodiccompartment. In other words, the electroplating cell (1) must preferablyalways be in hydraulic connection with the enrichment cell compartment(2) which is time by time anodically polarised, in order to guaranteethe self-regulation of the concentrations of all the species.

In stationary conditions, a simple regulation of the excess current ofthe enrichment cell (2), requires the passage of a hydrogen ion molethrough the cation-exchange membrane (6) for each mole of H⁺ionsgenerated at the anode (5), in order to perfectly balance the acidity ofthe system and automatically restore the M^(z+)ions concentration. Inparticular, for z moles of electrons transported in the electroplatingcell (1), it is simply necessary to apply a current sufficient toprovide for the passage of (1+t) ·z moles of electrons to the enrichmentcell (2), where the ratio between 1 and (1+t) is the hydrogen iontransport number (equivalent to the faradic efficiency), and the ratiobetween t and (1+t) is the transport number of the metal cation(parasitic current fraction). In stationary conditions, therefore, withthe passage of z moles of electrons in the electroplating cell (1) onemole of metal M is deposited onto the matrix (3) and z moles of H⁺arereleased at the insoluble anode (5): concurrently, in the enrichmentcell (2) the passage of (1+t)·z moles of electrons takes place with therelease of (1+t) moles of M^(z+)in the anodic compartment (10), thedeposition of t moles of M and the consumption of z moles of H⁺to formz/2 moles of hydrogen at the cathode (7) of the enrichment cell (2).Thus the cathodic compartment of the enrichment cell (2), is deputed tothe hydrogen discharge reaction on the surface of the cathode (7),according tozH⁺ +ze ⁻ →z/2H₂and to the metal deposition according totM^(z+) +t·z e ⁻ →tM

An immediate check of the balance of matter and of charge in thiscompartment shows how, by means of said half-reaction, for each mole Mof metal deposited on the cell (1) the consumption of z moles ofhydrogen ions transported through the cation-exchange membrane (6) isexactly effected.

Therefore, the above described process is self-regulating and itsoverall balance of matter implies only a consumption of watercorresponding to the quantity of oxygen released in the electroplatingcell and the quantity of hydrogen released in the enrichment cell: thewater concentration may be easily restored by a simple filling-up, forexample in the electroplating cell (1). In any case, this waterfilling-up does not imply any further complication of the process, as itis normal, in any electroplating process with consumable anode orinsoluble anode, evaporation phenomena lead per se to the need forcontrolling the water concentration by continuous filling-up. As thecation transport through the membrane (6) of the enrichment cell (2)usually takes place in the hydrated form, it is also possible that aslight concentration of the catholyte in the compartment (9) may berequired when the evaporation in this compartment is not sufficient tobalance said excess transported water.

The disclosed general scheme can be further implemented with otherexpedients known to the experts of the field, for instance by deliveringthe oxygen, which evolves at the anode (5) of the electroplating cell(1), to the cathodic compartment (9) of the enrichment cell (2), toeliminate the hydrogen discharge in the latter and depolarise theoverall process with back production of water; in this way a remarkableenergy saving is obtained as the electric current consumption imposed bythe process is only the amount necessary for the metal M deposition,whereas no overall consumption of water occurs.

The following examples intend to illustrate some industrial embodimentsof the present invention without however limiting the same thereto.

EXAMPLE 1

In this experiment, a steel sheet has been subjected to a tin platingprocess in an electroplating cell containing a bath of methansulphonicacid (200 g/l), bivalent tin (40 g/l) and organic additives according tothe prior art, employing as anode a positively polarised titanium sheet,coated with iridium and tantalum oxides, directed to the oxygenevolution half-reaction. An enrichment cell has been equipped with atitanium cathode in the form of a flattened expanded sheet provided witha conductive coating and a consumable anode of tin beads, confined bymeans of a positively polarised titanium expanded mesh basket providedwith an electrically conductive film. The exhaust electrolytic bath,recycled from the electroplating cell has been used as anolyte and amethansulphonic acid solution at low concentration of stannous ions, asthe catholyte. The catholyte and the anolyte of the enrichment cell havebeen divided by means of Nafion® 324 cation-exchange sulphonic membrane,produced by DuPont de Nemours, U.S.A.

Utilising a current density of 2.94 kA/m² in the enrichment cell, acontinuous tin plating of the steel sheet could be carried out for anoverall duration of one week, with a faradic efficiency of 94%, withoutany intervention besides the progressive water filling-up in theelectrolyte of the electroplating cell, monitored through a levelcontrol, and the forced evaporation in an auxiliary unit of a smallfraction of the catholyte, which received excess water due to thehydrogen ions transport migrating through the cation exchange membranewith their hydration shell.

After one week, a current reversal was effected on the enrichment cellfor 6 hours in order to dissolve the tin deposited at the cathode,reverting then to normal operation for another week, upon restoring thetin load in the anodic basket.

EXAMPLE 2

A steel wire was subjected to a copper plating process in anelectroplating cell containing a bath of sulphuric acid (120 g/l),cupric sulphate (50 g/l) and organic additives according to the priorart, using as the anode a positively polarised titanium sheet, coatedwith iridium and tantalum oxides, deputed to the oxygen evolutionhalf-reaction.

An enrichment cell, fed at the anodic compartment with the exhaustelectrolytic bath coming from the electroplating cell, has been equippedwith an AISI 316 stainless steel cathode and a consumable anode ofcopper shavings, confined by means of a positively polarised titaniummesh basket provided with a conductive coating and enclosed in a highlyporous filtering cloth. As the catholyte a sulphuric solution with a lowconcentration of copper ions has been used. The catholyte and theanolyte of the enrichment cell have been divided by means of a sulphoniccation exchange membrane, Nafion® 324 produced by DuPont de Nemours,U.S.A. Utilising a current density of 4.55 kA/m² in the enrichment cell,a continuous copper plating of the steel wire could be carried out foran overall durabon of one week with a faradic efficiency of 88%, withoutany intervention besides the progressive water filling-up in theelectroplating cell, monitored through a level control. After one week,a current reversal was effected on the enrichment cell for 6 hours inorder to dissolve the copper deposited at the cathode, reverting then tonormal operation for another week, upon restoring the copper load in theanodic basket.

In the description and claims of the present application, the word“comprise” and its variation such as “comprising” and “comprises” arenot intended to exclude the presence of other elements or additionalcomponents.

1. A self-regulating process for restoring the concentration of a metaland the acidity of an acid electrolytic bath coming from at least oneelectroplating cell where said metal is plated on a conductivenegatively polarized matrix while oxygen and acidity are generated at apositively polarized insoluble anode, carried out in at least oneenrichment cell comprising an anodic compartment and a cathodiccompartment separated by a cation exchange membrane, the anodiccompartment comprising a soluble anode made of the metal to be platedand the cathodic compartment comprising a cathode made of a corrosionresistant material, the at least one electroplating cell and the atleast one enrichment cell being hydraulically connected, the acidelectrolytic bath containing the metal to be plated being recirculatedfrom the anodic compartment of the at least one enrichment cell to theat least one electroplating cell, the at least one electroplating celland the at least one enrichment cell being respectively supplied with anelectroplating current and an enrichment current, wherein the ratiobetween the enrichment current and the electroplating current is thereciprocal of the current efficiency of the enrichment cell expressed asthe hydrogen transport number, and wherein only the water consumed byelectrolysis or evaporation is restored, the balance of matter of theremaining species being self-regulated.
 2. The process of claim 1wherein said metal to be plated has an oxidation potential more positivethan that of hydrogen.
 3. The process of claim 2 wherein said metal iscopper.
 4. The process of claim 1 wherein said metal to be plated has ahigh hydrogen overpotential.
 5. The process of claim 4 wherein said highhydrogen overpotential metal is selected from the group consisting ofzinc, tin and lead.
 6. The process of claim 1 wherein the polarity ofthe anodic compartment and of the cathodic compartment of the enrichmentcell is periodically reversed.
 7. The process of claim 1 wherein theratio between said hydrogen transport number and the transport number ofthe cations of said metal to be plated is comprised between 85:15 and98:2.
 8. The process of claim 1 wherein the oxygen formed at thepositively polarized insoluble anode of at least one electroplating cellis bubbled into the cathodic compartment of the at least one enrichmentcell.